The present invention relates to a method of and apparatus for cooling and guiding metal strips cast by a continuous casting apparatus.
More particularly, the present invention is concerned with a method of and apparatus for cooling and guiding a metal strip which has been cast by and delivered from a casting mold of a continuous casting apparatus, at a straightening section connected to the latter.
More specifically, the method and apparatus of the invention are adapted for use in combination with a continuous casting apparatus of a type generally referred to as "wheel-belt caster" having a casting wheel provided with a peripheral groove adapted to be closed over a portion of its circumferential length by an endless belt, such that the endless belt and the peripheral groove define the casting mold in cooperation with each other.
A casting apparatus called "wheel-belt caster" has been well known as a kind of a continuous casting apparatus for producing continuous metal strips. This wheel-belt caster has a casting wheel provided with a peripheral groove, and a steel endless belt adapted to run in contact with a portion of the circumferential length of the groove, such that the peripheral groove and the steel endless belt in combination define a casting mold.
In operation, a molten metal is poured into the casting mold from an inlet of the latter where the endless steel belt comes into contact with the casting wheel, while a solidified metal strip is continuously delivered from an outlet of the casting mold where the endless steel belt leaves the casting wheel.
In this type of casting apparatus, a straightening section is disposed adjacent to the outlet of the casting mold, for the purpose of straightening the metal strip which assumes a curved configuration soon after the delivery from the casting mold.
The straightening work in this section is usually performed by pinch rollers disposed at the downstream side of the casting mold and adapted to forcibly pull out the solidified metal strip from the latter.
The specification of U.S. Pat. No. 3,659,643 discloses a continuous casting machine in which a casting shoe is used in place of the endless belt of the wheel-belt caster. In this casting machine, the straightening of the case metal strip is effected through forcibly guiding the metal strip by means of a knife-like guiding plate disposed adjacent to the outlet of the casting mold.
However, these conventional straightening methods have suffered various problems as explained below, especially in case of casting of metals having low thermal conductivities such as steel and the like. At the same time, it has been pointed out that, in case of the straightening method relying upon the knife-shaped guide plate, the edge and the guide surface of the guide plate is worn down impractically soon.
The inconveniences caused in the course of straightening of the metal strips having low thermal conductivities, cast by the aforementioned type of a continuous casting apparatus, are as follows.
When the metal to be straightened is, for example, copper whose thermal conductivity is as high as about 330 Kcal/mh.degree. C., the heat possessed by the strip is transferred to the mold walls in a relatively short time, and the strip has been solidified substantially to the core thereof when pulled out from the casting mold, so that the straightening can be carried out without substantial difficulty or problem. However, when the metal is such one as having a low thermal conductivity, e.g. a steel whose thermal conductivity is as low as 29 Kcal/mh.degree. C., the metal strip has been solidified only to the depth of about 10 mm or so from the surface, when it is pulled out from the casting mold. Thus, when delivered to the straightening section, only the surface portions of the strip have been solidified to form thin solidified shells, while the inner part of the same is still in molten condition.
When the straightening is effected on such a metal strip, strain is generated in the thin solidified shells to cause a cracking of the latter, resulting in an accident so-called "break-out" in which the inner molten metal flows out of the solidified shell through the cracks. The continuous casting work has to be stopped, once the break-out takes place. Conventionally, as a countermeasure for preventing the break-out, spray cooling has been effected on the metal strip pulled out from the casting mold, in which the metal strip is cooled by a coolant sprayed from a spray nozzle. However, unfortunately, this countermeasure is not effective enough to completely exclude the problem of break-out. As to the detail of this spray cooling technic, a reference shall be made to the specification of U.S. Pat. No. 3,623,535, although the latter is concerned with a technical subject entirely different from that of the present invention.
In the continuous casting of metal strips, it is essential to avoid the break-out attributable to the surface cracking of the metal strips, as will be realized from the foregoing description.
Besides that, there has been another problem of internal cracking which is often experienced in the course of the straightening. Although this internal cracking does not directly lead to a serious accident, this quality of the strip is often deteriorated fatally due to the presence of large internal cracks. In such a case, the strip cannot be sent to the next step of process, and has to be scrapped as being unacceptable. Practically, when the internal cracking is not so serious, the metal strip is subjected to a rolling before being sent to the cutting step, so that the small cavity caused by the internal cracking may be refilled.
This rolling step can be dispensed with, if the internal cracking during the straightening is completely avoided or, even if not, negligibly small. In addition, the use of the wheel-belt caster can be extended to the production of high quality steels in which the internal cracking is strictly prohibited.
However, no effective countermeasure has been taken or proposed to avoid the internal cracking, because the cause of the internal cracking has not been clarified.